Yes, a score of 4 is significant. Yes, a dairy allergy or sensitivity can cause sinus congestion/mucus. Dairy can do that even in the absence of an allergy, though...
The best way to know for sure is to avoid dairy for a few weeks and see if your symptoms clear up. Then you can try some dairy again and see if symptoms return.
Unfortunately, not many doctors really understand food allergies all that well...
Chronic Lyme, Bart., Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication/Chemical Allergies & Intolerances, Asthma, Gut dysmotility & non-specific inflammation, Lupus, Osteoporosis, etc.; G-Tube
Meds: Flagyl, Domperidone, Zofran, Claritin, Andrographis & other herbs, homeopathy, supplements, etc.